mmarshall

Yes...basically you hit it. Full-lock differentials, like traditional part-time 4X4 aren't considered car-based. They lock up all four wheels at the same speeds...which precludes their use on dry pavement in anything but an arrow-straight line, because, in a curve, the outside wheels want to roll around the curve faster than the inside ones, causing tire-scrubbing, drive-line windup, and other damage. True, some car-based systems have push-button locking diffs, but they still require driver-action (and you must remember to disengage them once you are on dry pavement, or risk the problems I just described). The maddening part with part-time or locking-diff systems is when the pavement is partially-wet, partially-frozen, and partially-dry, switching back and forth as you drive down the road (which often happens after winter-roads are plowed and treated). You never really know exactly what you have on the road at any moment under the wheels, or if the lock-up system can be safely used. That, BTW, is one of the main reasons why Subaru and Audi did so much to perfect car-based full-time AWD (though its first use was actually with the 1980-81 AMC Eagle)....just get in, drive, and don't worry about the road surface, drivetrain or any specific driver-input.

bruce van

Nice video. I've got a 2011 Outback and it has never let me down on some very rough roads. I've taken it on roads where you only see lifted trucks, jeeps, and Range Rovers.

The Outback is by far the most capable, well rounded car I've owned.

mmarshall

Thanks.

Yes....agreed. My 7-year-old Outback, like yours, would go through just about anything short of Jeep-Wrangler stuff. I would have replaced it with another one except that there were some things I didn't like about the newer ones....chiefly, the CVT used with the 2.5L models and the loss of the coolant-temperature-gauge, using red and blue idiot-lights instead.

UberNoob

and i think RAV4 would be a better setup since you get to choose when to turn it on and off to save gas when you dont need it

mmarshall

Turning it on and off (a least as I understand it) only affects the AWD lock-up differential, not the 2WD/AWD itself. But, if wrong, I'll stand corrected.

KrazyLexus

How do you explain this then?

mmarshall

I didn't say that the SH-AWD system can't get up slick hills. My point is that it's not necessarily because of the unique side-to-side torque distribution. Subarus, for example, do it without that particular feature.

Och

iTrader: (3)

[quote=mmarshall;7486737]Mike, no offense, but I don't think you have in-depth understanding of how different AWD systems work. And unfortunately, most auto makers do not bother specifying details about their AWD systems on various models - just slapping AWD label on the car.

Some AWD systems are completely useless - if all three diffs are open. Put one of the wheels of such car on a patch of ice and it won't go anywhere. Then there are AWD systems where all three differentials are limited slip - and those of course are much better. Of course, there are different types of LSDs, also tuned to allow different % of slip, and working in combination with traction/stability systems can become very capable, but still no match to full 4wd lock.

With electronic torque vectoring systems, such as SH-AWD, its much more than varying torque from side to side. These systems can send as much as 70% of engine's torque to any one wheel, independent from other wheels. Most LSD's can only send 5%-20%, depending on how they are setup. Also, these systems are doubling down as stability systems, and as a result vehicles equipped with them have incredible handling - Nissan GTR has no equal in it's class. Its almost like having four individual throttle and brake pedals, and having 8 feet to operate them.

It's a shame that Acura is abandoning its SH-AWD system. The new RDX no longer has it, the next RL is going to have a system that has nothing to do with SH-AWD, and I predict future TL and MDX will drop it as well. Fortunately other manufacturers (including Subaru) are starting to utilize similar systems.

mmarshall

Yes and no. I know most of the basics, but not all of the details of the innermost workings of all the parts. To do so requires a degree in powertrain-engineering or a company-training course for Certified Technicans. I haven't been through that kind of detailed technican-like training...but I don't feel it is generally necessary for the type of auto-reviews and general auto-discussion that we do here on CAR CHAT. Nor is it necessary on your part, either.


Subaru used to have an excellent 3-D cutaway display at the auto shows that had clear engine/transmission/differential covers, and showed all the mechanical-workings of a typical Forester AWD system spinning away under the clear covers. It was a very impressive display (and no doubt helped generate a lot of potential sales). Haven't seen it, though, for the past few years.....my guess is that it might have been dropped from cost-cutting.

They also used to have an excellent 3-D video on their website on how the five different Subaru AWD systems worked...they depended on engine/transmission. There were both vicsous-fluid and electronic clutch-pack center-diffs, open and LSD rear-diffs, and the special driver-adjustable center-differential on the STi....somewhat similiar to that on the Mitsubishi Evo.


Assuming that the ice has even slickness and low-traction level, that's probably true.

The Mercedes Gelandewagen (G-Wagon) goes one better even than that...three fully-locking diffs; front, center and rear. Hard to beat off-road traction like that.

Of course, there's a reason for that. it was originally develped as a German/Austrian military vehicle, with a requirement, like a Hummer H1, to go through almost anything.

Yes, you are correct. I'm wasn't contesting that....although maybe I could have made my comment on side-to-side vs. front/rear a little clearer.


The new RDX, from Acura's own comments and press-releases, is not meant to be anywhere near as sport-oriented and handling-precise as the old model was......there were a number of customer complaints on its overly-stiff suspension and ride (I noted the quite stiff ride myself in my RDX review). Therefore, Acura felt that the new version didn't need the handling advantages of the SH-system, and apears to be setting for a simpler system with less cost. The punchy turbo-4 on the old model (one of the very few Honda/Acura fours with any real low-end torque) has also been dropped in favor of a smoother, more refined but less-punchy normally-aspirated V6. The new version may not appeal as much to enthusiasts and aggressive drivers, but will doubtless appeal more to mainsteam-buyers.

KrazyLexus

Good info and testing. Wish they had teh SH-AWD though in this.

Och

iTrader: (3)

Subaru and Jeep generally have pretty good, in depth, specifications of their AWD systems - since they are "AWD-oriented" manufacturers, for a lack of better term. However, many other manufacturers often don't bother explaining how their systems work.


The Gasenwagen isn't the only SUV that has three fully locking diffs - there are cheaper options out there. A Jeep Wrangler Rubicon has that capability and costs less than half of the German, and Soviet UAZ Hunter can be had for 1/10 of the price.

tex2670

Isn't there a split surface, enabling the SH-AWD--right side has better grip than left?

UberNoob

there is a common that youtube video page claiming that Acura was using winter tires on the MDX while theyve kept all seasons if not even summer tires on the competitors

not sure if it is true, more 3rd party testing is probably need to prove this video's non-partiality

Och

iTrader: (3)

I doubt thats true, plus in that particular test tires would make very little difference. One side was on ice - where even winter tires would slip almost as much as all seasons, and the other side was on dry pavement, where winter tires might actually be at a disadvantage.

UberNoob

well depends on whether the tires were studded or not
that would make a difference